Dynamics of Electron Injection in Nanocrystalline Titanium Dioxide Films Sensitized with [Ru(4,4'-Dicarboxy-2,2'-Bipyridine)2(NCS)2] by Infrared Transient Absorption

We have used femtosecond pump-probe spectroscopy to time resolve the injection of electrons into nanocrystalline TiO₂ film electrodes under ambient conditions following photoexcitation of the adsorbed dye, [Ru(4,4′-dicarboxy-2,2′-bipyridine)₂(NCS)₂] (N3). Pumping at one of the metal-to-ligand charge-transfer absorption peaks and probing the absorption by injected electrons in the TiO₂ at 1.52 μm and in the range of 4.1-7.0 μm, we have directly observed the arrival of electrons injected into the TiO₂ film. Our measurements indicate an instrument-limited ∼50 fs upper limit on the electron injection time. We have compared the infrared transient absorption for noninjecting systems consisting of N3 in ethanol and N3 adsorbed to films of nanocrystalline Al₂O₃ and ZrO₂ and found no indication of electron injection at probe wavelengths in the mid-IR (4.1-7.0 μm).